Various noninvasive or minimally invasive methods to measure analytes, particularly glucose, have been described. For example, March, U.S. Pat. Nos. 3,958,560 and 4,014,321, discloses a glucose sensor wherein a patient's eye is automatically scanned using a source of light at one side of the cornea. A sensor located at the other side of the cornea detects the light that passes through the cornea. The level of glucose which rotates the plan of polarized light in the aqueous humor of the patient is a function of the amount of radiation detected. However, this sensor system is not necessarily specific or widely applicable to detection of analytes other than glucose, because it does not exploit the use of biological molecules which can detect glucose or other analytes in a body tissue or fluid sample. Biological molecules, as is well known, can provide very specific and sensitive detection reagents for particular analytes.
Schultz, U.S. Pat. No. 4,344,438, discloses a system for monitoring low molecular weight compounds in blood plasma by optical means, which involves a chamber which contains specific receptor sites for the plasma constituent to be analyzed. This system is very invasive, however, because it must be implanted within the blood stream using a hypodermic needle. The system also inherently contains the risks of clotting around the device, obstruction, and other adverse reactions, including immune reactions, general irritation, and foreign body reactions.